Method for operating a brushless direct current motor

ABSTRACT

The invention relates to a brushless direct current motor with a synchronous motor, which is supplied with a direct current voltage as the input voltage for commutating the motor phases. The speed of the synchronous motor is modified or varied in two speed ranges. In the first speed range, a linear change of the input voltage is carried out up to a speed threshold value. In the second speed range with higher speeds than in the first speed range, a vector modification of the input voltage is carried out, for example for controlling the speed of a fan motor.

[0001] Electrical drive units are used in a multitude of applicationareas. For example, in motor vehicles different movable components ofthe motor vehicle may be driven by electrical drive units, for exampleseats, window lifters, sun roofs, and so forth. Alternatively, thesecomponents can be operated with a variable r.p.m., for exampleventilators. Electrical drive units comprise an electric motor forproducing and providing of an electrical power output and a controlmodule for controlling and monitoring the electric motor, for examplefor controlling the r.p.m. and power of the electric motor in closedloop fashion. The electric motors may be constructed as a.c. motors,that is either asynchronous motors operating asynchronously to the inputfrequency, or as synchronous motors operating synchronously with theinput frequency with an external commutation of the motor phase windingsfor stepping the motor winding. Alternatively, the motors may beself-commutating d.c. motors, depending on a applied input voltage. Ifthe commutating of the motor phase windings of permanent excitationsynchronous motors is dependent on a position recognition, morespecifically if the commutation takes place depending on the directlymeasured position of the rotor or on the rotor position determined fromother motor characteristics, then the synchronous motors are operatedjust like electronically commutated that is self commutating d.c. motors(EC-DC motors). More specifically, the self commutating of thesynchronous motor takes place depending on the input voltage applied tothe individual windings of the synchronous motor. Such commutatedsynchronous motors are referred to as brushless d.c. motors.

[0002] In connection with r.p.m. controlled electrical drive units withbrushless d.c. motors the variation of the r.p.m. of the synchronousmotor is achieved by varying the amplitude of the d.c. input voltageapplied to the individual windings of the synchronous motor during thecommutation of the motor phase windings. The variation of the amplitudeof the d.c. input voltage takes place, as a rule, through a clockedoperation that is, for example, realized by means of pulse widthmodulation, of the commutation switches that perform the commutation ofthe motor phase windings. These commutation switches are particularlyconstructed as commutation transistors. More specifically, thecommutation switches or commutation transistors are used for thecommutation of the motor phase windings and for the clocking of theinput voltage and thus for the variation of the amplitude of the appliedinput voltage and thus of the motor voltage.

[0003] In this connection it is disadvantageous that due to theswitching transients caused by the clocked operation of the commutationswitches or commutation transistors the current flow in the supplyconductor for supplying the input voltage or d.c. voltage isperiodically and abruptly interrupted and switched on again. Suchswitching transients have strong reaction effects on the supplied inputvoltage or d.c. voltage. In order to limit these adverse reactionsduring the operation of the brushless d.c. motor a large effort andexpense is required for additional structural components such as EMVcoils, storage capacitors, interference suppression capacitors, chokesand so forth, whereby substantial costs and a large space requirementare caused.

[0004] It is the object of the invention to provide a method foroperating a brushless d.c. motor according to the preamble of patentclaim 1 by means of which a simple reliable and cost effective operationof the brushless d.c. motor is made possible. This object is achievedaccording to the invention by the features of the characterizing clauseof patent claim 1.

[0005] Advantageous further developments of the invention are providedby the further patent claims.

[0006] According to the present method for varying the r.p.m. of thesynchronous motor the d.c. voltage of the brushless d.c. motor and thusthe input voltage that is supplied to the synchronous motor is linearlycontrolled or continuously varied in closed loop fashion in a firstr.p.m. range with low r.p.m.s. More specifically, the exact momentarilyrequired motor voltage is supplied as input voltage to the synchronousmotor. The different between the motor voltage and the maximum inputvoltage is transformed into a dissipated power. Upon reaching an r.p.m.threshold value that depends on a predetermined voltage threshold valueof the input voltage, the d.c. voltage is controlled in a closed loopmanner in a second r.p.m. range that follows the first r.p.m. rangetoward higher r.p.m.s by a shifting of the vectors of the motor voltage(field orientation). More specifically, the voltage angle between thed.c. voltage that is applied to the respective motor winding of thesynchronous motor of the brushless d.c. motor, namely the outer motorvoltage of the synchronous motor or rather the motor terminal voltage,and the voltage at the rotor of the synchronous motor, namely the innermotor voltage of the synchronous motor or the EMK (electromotive force)or the revolving field voltage is rotated. Accordingly, the inputvoltage supplied to the synchronous motor for varying the r.p.m. iscontinuously changed in the first r.p.m. range by means of a closed looplinear control by adjusting the amplitude of the motor terminal voltage.This input voltage is supplied to the commutation switches or to thecommutation transistors for commutating the motor phase windings of thesynchronous motor. The variation in the first r.p.m. range takes placeby a closed loop linear control by adjusting the amplitude of the motorterminal voltage to continuously vary the motor terminal voltage. In thesecond r.p.m. range the motor terminal voltage is controlled by means ofa closed loop vector control through the field orientation by increasingthe effective motor current of the synchronous motors and thusincreasing the motor torque.

[0007] Particularly, the first r.p.m. range is extended by the linearclosed loop control of the input voltage or motor terminal voltage, to avoltage threshold value that determines the r.p.m. threshold value atwhich the synchronous motor reaches its maximal motor terminal voltage.More specifically, the linear control of the input voltage extends to apoint where the motor terminal voltage of the synchronous motor is equalto the maximum input voltage of the brushless d.c. motor, for examplewhere it is equal to the operating voltage of the brushless d.c. motor.Thus, in the second r.p.m. range where the motor terminal voltage of thesynchronous motor has a maximum and constant amplitude, the effectivemotor current of the synchronous motor is increased by the closed loopvector control and thus the motor torque is raised. By suitablydimensioning the brushless d.c. motor or rather the synchronous motor,one achieves a large second r.p.m. range compared to the first r.p.m.range. That means that the r.p.m. threshold value which is predeterminedby the voltage threshold value is small. For this purpose it ispreferred to use a synchronous motor having a high number of windingswhich motor reaches its maximum motor terminal voltage already at lowr.p.m.s. The production of a closed loop controlled d.c. voltage asinput voltage in the first r.p.m. range can be realized either by aclosed loop linear control of the commutation switches or commutationtransistors or it can be realized by a closed loop control elementconnected in series with the brushless d.c. motor and forming part of acontrol module of the electrical drive unit. More specifically, theclosed loop control element is either a separate closed loop controlunit connected in series with the brushless d.c. motor or thecommutation switches or commutation transistors take over the functionof the closed loop control unit. Preferably, the linear closed loopcontrol of the input voltage in the first r.p.m. range is performed by adetermined number of commutation switches or commutation transistors,which are linearly controlled by a closed loop control, whereby for eachcontrol phase of the synchronous motor either only one respective switchor transistor is controlled or all commutation switches or commutationtransistors of the synchronous motor are controlled in a linear closedloop fashion. The respective commutation switch or commutationtransistor is the one that is used for commutating the motor phasewindings in the control phase. Similarly, the controlled commutationswitches or commutation transistors are the ones used for thecommutation of the motor phase windings in the control phase of thesynchronous motor. Normally, two commutation switches or two commutationtransistors are provided in a brushless d.c. motor for each controlphase of the synchronous motor for commutating the motor phase windings.The first r.p.m. range extends only to relatively low r.p.m.s of thesynchronous motor where the motor current of the synchronous motor isstill low. Therefore, the dissipation power is also low. The dissipationpower is the product of the motor current of the synchronous motor and avoltage difference. The voltage difference is the difference between theinput voltage of the brushless d.c. motor and the motor voltage or motorterminal voltage of the synchronous motor. This voltage differenceoccurs as a voltage drop across the closed loop control element. Thecontrol element is either part of the linear closed loop control unitconnected in series or the commutation switch itself.

[0008] Due to the combination of the linear closed loop controlledoperation of the brushless d.c. motor at low r.p.m.s of the synchronousmotor with the field orientation operation of the brushless d.c. motorat higher r.p.m.s of the synchronous motor, the linear closed loopcontrol takes place in a range with low motor currents of thesynchronous motor. Hence, dissipation losses are low. This appliesparticularly when using the brushless d.c. motor in blowers or pumps.Thus no interfering effects occur in the operation of the brushless d.c.motor. As a result, no over-dimensioned or additional structuralelements are required as is the case when high dissipation power lossesoccur.

[0009] Since the commutation switches or commutation transistors are notclocked (not operating in a switching operation) and therefore no oronly small disturbances occur in the supply conductor for feeding theinput voltage, EMV-measures can be avoided or can be substantiallyreduced.

[0010] If in the first r.p.m. range the commutation switches or thecommutation transistors themselves are controlled linearly in closedloop fashion, no additional power structural components or adjustmentmembers are needed for the linear closed loop control. This is sobecause in the first r.p.m. range only a small dissipation power occursdue to the small motor currents of the synchronous motor. Thus,commercially available commutation switches or commutation transistorsmay be used.

[0011] The method will be described in the following in more detail withreference to an example embodiment and in connection with the drawing,whereby

[0012]FIG. 1 shows a schematic block diagram of the essential componentsof the brushless d.c. motor;

[0013]FIG. 2 shows the curve of the motor voltage and the motor currentof the synchronous motors of the brushless d.c. motor as a function oftime during the commutation or energizing of the motor windings of thesynchronous motor; and

[0014]FIG. 3 shows the characteristic of the r.p.m. of the synchronousmotor of the brushless d.c. motor depending on the voltage.

[0015] According to FIG. 1 a variable input voltage U_(E) is supplied asa d.c. voltage through the supply conductor 11 to the brushless d.c.motor 1 for varying the r.p.m. N_(M) of the synchronous motor 10. Theinput voltage U_(E) is obtained from the supply voltage U_(B) of thebrushless d.c. motor 1. For example the supply voltage U_(B) is 13.5volts. The brushless d.c. motor 1 comprises six commutation transistors3 to 8 operated in a bridge circuit as commutation switches forcommutating the motor phase windings of the synchronous motor 10. Twocommutation transistors 3, 4; 5, 6; 7, 8 are provided for each controlphase for commutating the motor phase windings of the synchronous motor10. A position indicator 9 evaluates the motor position, or rather theposition of the synchronous motor 10. For example, three Hall sensorsgenerating a digital output signal may be provided as the positionindicator 9. The output signal of the position indicator 9 is suppliedto a commutating logic circuit 2 which triggers the commutationtransistors 3 to 8 depending on the instantaneous motor position of thesynchronous motor 10 to thereby commutate the motor windings of thesynchronous motor 10.

[0016] Hereto FIG. 2 illustrates as a function of time the commutationsequence of the synchronous motor 10 which is energized in three triggerphases by a three-phase current. FIG. 2 shows the motor voltage which isthe motor terminal voltage U_(M) applied to the windings of thesynchronous motor 10, and the motor current I_(M) of the synchronousmotor 10. The commutation of the motor phase windings of the synchronousmotor 10 is performed cyclically respectively in 60° steps followingeach rotation of the synchronous motor 10 as recognized by the positionindicator 9. The commutation is performed at commutation points of timewhen the motor terminal voltage U_(M) has reached its respective maximumvalue where it corresponds approximately to a d.c. voltage. Thecommutation points of time are predetermined by the commutation logic 2.The commutation is realized by the switching of the commutationtransistors 3 to 8. The commutation points of time for switching on themotor current I_(M) in the respective motor phase winding of thesynchronous motor 10 are the points of time t1, t3, t5, t7, t9 . . . forthe positive half wave of the motor terminal voltage U_(M). Thecommutation points of time for the negative half wave of the motorterminal voltage U_(M) are t2, t4, t6, t8, . . . This switching avoids awaviness in the current characteristic of the total current formed asthe sum of the motor currents I_(M) of the individual motor phasewindings. As a result, an approximate d.c. current is present in thesupply conductor 11 and thus no current peaks occur.

[0017]FIG. 3 illustrates the characteristic of the motor voltage ormotor terminal voltage U_(M) as a function of the r.p.m. n_(M) of thesynchronous motor 10. For varying the r.p.m. n_(M) in a first r.p.m.range 12 up to the r.p.m. threshold value n_(S), the motor terminalvoltage U_(M) present at the motor windings of the synchronous motor 10is varied by a linear closed loop continuous control. Thus, in the firstr.p.m. range the r.p.m. of the synchronous motor 10 is a function of themotor terminal voltage U_(M). In the second r.p.m. range 13 startingwith the r.p.m. threshold value n_(S) up to the nominal value nN of ther.p.m. n_(M) of the synchronous motor 10, the inner motor voltage or thepole wheel voltage or the EMK (electromotive force) is controlled by aclosed loop control of the field orientation while keeping the motorterminal voltage U_(M) constant. Stated differently in the second r.p.m.range 13 the r.p.m. is a function of the voltage between the constantmotor terminal voltage U_(M) and the inner motor voltage or pole wheelvoltage or EMK of the synchronous motor 10. More specifically, ther.p.m. threshold value n_(S), is defined as that r.p.m. n_(M) of thesynchronous motor 10 at which the input voltage U_(E) and thus the motorterminal voltage U_(M) of the synchronous motor 10 achieves the value ofthe supply voltage U_(B) that is the motor terminal voltage U_(M)achieves the maximum value U_(M Max). This maximum value U_(M Max) ofthe motor terminal voltage U_(M) is achieved already at small r.p.m.sn_(M) of the synchronous motor 10 by a suitable dimensioning of thesynchronous motor 10. More specifically, the r.p.m. threshold valuen_(S) is accordingly small so that the second r.p.m. range 13 with theclosed loop control of the field orientation also begins already atsmall r.p.m.s n_(M) of the synchronous motor 10.

[0018] For example, if it is intended to vary the r.p.m. n_(M) of thesynchronous motor 10 of a brushless d.c. motor 1 constructed as a blowermotor, in an r.p.m. range between 400 r.p.m. and 2400 r.p.m., whereinthe latter is the nominal r.p.m. n_(N), then upon reaching the nominalr.p.m. n_(N) of, for example 2400 r.p.m., the defined or rated poweroutput of the synchronous motor 10 or rather of the brushless d.c. motor1 is produced. The synchronous motor 10 comprises, for example a maximuminput power of 400 W at a motor terminal voltage U_(M) of 13.5 V andthus a maximum motor current I_(M) of 30 A. This motor current isdistributed by means of six commutation transistors 3 to 8 onto themotor windings of the synchronous motor 10. Due to the nonlinear loadcharacteristic of the blower the maximum dissipation power, for exampleabout 45 W, occurs during the linear closed loop control at 70% of thenominal r.p.m. n_(N), for example at 1680 r.p.m. In case the commutationtransistors 3 to 8 themselves are used for the linear closed loopcontrol, the dissipation power is distributed onto these commutationtransistors 3 to 8. As a result, the maximum dissipation power per eachof the commutation transistors 3 to 8 is 7.5 W, provided all sixcommutation transistors functions as a closed loop control element. Ifonly three commutation transistors function as closed loop controlelements, the dissipation power per control element is 15 W. Commercialtransistors having an ON resistance of, for example 12 mΩ, have,however, in a switching operation at the maximum current of the motorcurrent I_(M) of 30 A a permissible dissipation power of 3.6 W. Thus, anover-dimensioning of the commutation transistors 3 to 8 would becomenecessary.

[0019] However, the maximal dissipation power is reduced to 10 W atabout 40% of the nominal r.p.m. n_(N), for example 960 r.p.m. due to thesmall range of the linear closed loop control, if the followingconditions are satisfied. First, a linear closed loop operation at smallr.p.m. n_(M) is combined with a field orientation operation at higherr.p.m.s n_(M) for the above described synchronous motor 10 having amaximum input power of 400 W. Second, the synchronous motor 10 is soconstructed, for example by increasing the number of the motor windingsby, for instance 50%, that the synchronous motor 10 achieves the maximumU_(M MAX) of the motor terminal voltage U_(M) already at 50% of thenominal r.p.m. n_(N) of, for example, 2400 r.p.m., that is at 1200r.p.m. Third, the supply voltage U_(B) of, for example 13.5 V is equalto the motor terminal voltage U_(M). Fourth, the variation of the r.p.m.n_(M) in the second r.p.m. range is achieved through the fieldorientation, more specifically through the adjusting of the anglebetween the motor terminal voltage n_(M), which is the maximal motorterminal voltage n_(M MAX), and the inner voltage, namely the pole wheelvoltage or the EMK (electromotive force) of the synchronous motor 10. Ifthe commutation transistor 3 to 8 are again used themselves for thelinear closed loop control, this dissipation power is distributed ontothe commutation transistors 3 to 8. As a result, the maximal dissipationpower of each commutation transistor 3 to 8 amounts only to 1.7 W,provided all six commutation transistors functions as closed loopcontrol elements. The dissipation power is only 3.3 W in case only threecommutation transistors function as closed loop control element. Thismaximal dissipation power is smaller than the permissible dissipationpower of 3.6 W of conventional transistors having an ON resistance of,for example 12 mΩ and the maximal current of the motor is 30 A.

[0020] Due to the combination of the linear closed loop operation withthe field orientation operation it is possible to avoid a clockedoperation of the commutation transistor 3 to 8, whereby no rapidswitching operations take place and thus interference suppressionmeasures are not necessary.

1. Method for operating a brushless d.c.-motor (1) with a synchronousmotor (10) which is supplied with a d.c. voltage as an input voltage(U_(E)) for commutating the motor phase windings, characterized byperforming a linear closed loop control of the input voltage (U_(E)) ina first r.p.m.-range (12) up to an r.p.m.-threshold value (n_(S)) forvarying the r.p.m. (n_(M)) of the synchronous motor (10), and, startingwith the r.p.m.-threshold value (n_(S)), performing a vector closed loopcontrol of the input voltage (U_(E)), in a second r.p.m.-range (13)having a higher r.p.m. (n_(M)) and following said first r.p.m.-range(12) of the synchronous motor (10).
 2. Method of claim 1, characterizedin that the r.p.m.-threshold value (n_(S)) is first set in accordancewith a voltage threshold value that depends on the supply voltage(U_(B)) of the brushless d.c.-motor (1).
 3. Method of claim 2,characterized in that the maximum motor terminal voltage (U_(M,MAX)) ofthe synchronous motor (10) is determined as the voltage threshold valuewhich in turn determines the r.p.m.-threshold value (n_(S)) and whereinsaid voltage threshold value is determined by the supply voltage (U_(B))of the brushless d.c.-motor (1).
 4. Method of claim 3, characterized inthat the synchronous motor (10) is so constructed that it reaches itsmaximum motor terminal voltage (U_(M,MAX)) at a low r.p.m. (n_(M)). 5.Method of one of the claims 1 to 4, characterized in that thecommutation of the motor phase windings of the synchronous motor (10) isperformed by commutation switches.
 6. Method of claim 5, characterizedin that the commutation of the motor phase windings of the synchronousmotor (10) is performed by two commutation switches for each motor phasewinding.
 7. Method of claim 5 or 6, characterized in that in each of themotor phase windings at least one of the two provided commutationswitches is used for the linear closed loop control of the input voltage(U_(E)) in the first r.p.m.-range (12).
 8. Method of one of the claims 1to 6, characterized in that a closed loop control element is connectedin series with the brushless d.c.-motor (1) for the linear closed loopcontrol of the input voltage (U_(E)) in the first r.p.m.-range (12). 9.Method of one of the claims 5 to 7, characterized in that commutationtransistors (3 to 8) are used as commutation switches.